Concurrent measurement of soil radon, soil thoron and soil CO2 efflux is based on the method developed by Giammanco
et al. (Geochem. Geophys. Geosys., 8(10), Q 10001, doi:10.1029/2007GC001644, 2007). An empirical
relationship links the 222Rn/220Rn ratio to the CO2 efflux: deep sources of gas are characterized by high 222Rn
activity and high CO2 efflux, whereas shallow sources are indicated by high 220Rn activity and relatively low CO2
efflux. This relationship is more constraining on the type and depth of the gas source than using the 222Rn/220Rn
ratio alone.We studied the temporal variation of the ratio between CO2 efflux and (222Rn/220Rn), that we define as
a Soil Gas Disequilibrium Index (SGDI). Since June 2006, periodical measurements of the SGDI were carried out
in ten sites located on the flanks of Mt. Etna, with sampling frequency of about ten days. Remarkable variations
in this parameter were recorded during the period 2006-2008 likely associated with changes in the activity level of
Mt. Etna. In particular, one of the sites located in the area called Primoti (on the lower east flank of the volcano) has
shown significant anomalous changes of the SGDI in time, possibly correlated with the eruptive/tectonic activity.
For this reason, in this site we set up an automatic monitoring station made of a Radon/Thoron monitor (model
RTM 2100, SARAD GmbH, Germany) coupled with a soil CO2 efflux station (model ACE, ADC BioScientific
Ltd., UK). The sampling frequency was set at 30 minutes, in order to allow for a sufficient decay equilibration in
the radon isotopes. Air temperature and barometric pressure were recorded as well, with the same sampling rate
as for the soil gases. The site chosen for testing the monitoring station is located on the east flank of Mt. Etna
at an altitude of about 520 a.s.l., in an area known for widespread diffuse emissions of CO2 and other gases of
magmatic origin. The preliminary data acquired so far showed an average soil CO2 efflux of 10 g m􀀀2 d􀀀1 (std
dev of about 7 g m􀀀2 d􀀀1) and average 222Rn and 220Rn activities of about 3.3 103 Bq/m3 (std dev of about
1140 Bq/m3) and about 2.0 103 Bq/m3 (std dev of about 620 Bq/m3), respectively. The corresponding values of
the SGDI thus obtained varied in the range from about -1.5 to about 70.1, with an average of about 7 and standard
deviation of about 6.3. The apparent baseline of the parameter is around the value of 3, and daily variations are
clearly detected due to the combined influence of air temperature and barometric pressure. No clear influence from
rainfall was observed. Some spikes were also detected, whose origin has to be studied by correlating the SGDI
with other environmental parameters as well as with changes in the volcanic/tectonic activity of Mt. Etna.